Okay... Now I've done some more testing and logging.
Here are the results.

For every Test-Case I will attach four files:
1. nvidia-bug-report from starting with normal running-script (tries to start gdm)
2. nvidia-bug-report from starting with startx -- -verbose 6 -logverbose 6
3. Xorg.0.log with the same conditions as 1
4. Xorg.0.log with the same conditions as 2